The research objectives of this proposal are to provide a detailed biochemical description of the mode-of-action of the seco-steroid vitamin D and its two daughter metabolites, 1,25-dihydroxyvitamin D (1,25(OH)2D3) and 24,25-dihydroxyvitamin D (24,25(OH)2D3). The major thesis of this study is that in terms of its chemical structure and postulated mode-of-action vitamin D is similar to that of other classical steroid hormones. Specific projects include: (a) Isolation of the mRNA for chick intestinal calcium binding protein, preparation of a cDNA copy, and by recombinant DNA techniques cloning of this cDNA so that ultimately the gene organization for CaBP can be studied and compared in the intestine and kidney of the chick and a number of other target organs for 1,25(OH)2D3; (b) isolation, purification and development of a radioimmunoassay to human placental CaBP; (c) isolation, purification and biochemical characterization of the chick intestinal receptor for 1,25(OH)2D3 through use of monoclonal antibodies; (d) development of an "exchange assay" for quantitation of intestinal receptor occupancy by endogenous 1,25(OH)2D3; (e) evaluation of chemical/biochemical structure-function relationships for vitamin D analogs which may function as either agonists or antagonists ("antivitamin") of 1,25-(OH)2D3 and its receptors; (f) to identify the pathways of inactivation or catabolism of 1,25(OH)2D3, and chemical characterization of any new metabolites discovered; (g) determination of vitamin D (and metabolite) mediated changes in intestinal membrane composition, organization and topology which may affect calcium and phosphorus translocation; (h) to identify biological responses uniquely mediated by 24,25(OH)2D3. These results will not only contribute to our biochemical understanding of vitamin D but also provide insights into the basis for pathophysiological dysfunction of the vitamin D endocrine system in a variety of human disease states including renal osteodystrophy, osteoporosis, sarcoidosis, and vitamin D resistant rickets.